Aircraft systems in general, and variable speed constant frequency electrical generating systems in particular, must be designed to survive short-term thermal overload conditions. This may be achieved by designing a cooling system that is sized for a worst case thermal overload condition but is significantly oversized for normal operating conditions. As thermal overload durations are short, lasting only a few seconds, a more efficient cooling system should involve sizing the system for a maximum long-term dissipation and should incorporate a thermal energy device to handle a short-term thermal overload condition.
Problems associated with a weight of a thermal energy device require the use of a phase change material. However, solid to liquid phase change materials which have a suitably high latent heat of fusion and melt at temperatures approximate to a maximum allowable temperature for a heat source have low thermal conductivities. Consequently, excessive temperatures develop at the heat source as large temperature differentials are required to drive the heat into the thermal energy device. The buildup of stress levels with each phase change resulting from melting and solidifying fronts moving in different directions further contributes to the imperfections present with solid to liquid phase change materials.
The desirability of removing heat from electronic equipment when temperatures outside of a missile envelope are lower than those of the electronic equipment is recognized in a patent to Basilius, U.S. Pat. No. 4,673,030, in which a unidirectional heat pipe and a phase change material package are employed. When the temperature exterior to the missile envelope is greater than that of the electronic equipment, the heat therefrom is conducted to a package containing phase change material which absorbs the heat from the electronic equipment, at which time the heat pipe acts as a thermal insulator. The patent to Basilius does not, as the invention to be described more fully hereinafter, address the operation of a thermal energy device except to mention various candidate solid to liquid phase change materials.
The problem of removing heat from electronic equipment is also discussed in a patent to Feldmanis, U.S. Pat. No. 3,651,865. The Feldmanis patent teaches an electronic equipment mounting plate with capillary heat pipes secured thereto. A condenser section is provided near the center of the mounting plate for a coolant to pass through cooling tubes positioned adjacent a central portion of the heat pipes to provide a heat sink. The patent to Feldmanis does not, as the invention to be described more fully hereinafter, provide for pumping a fluid to a location of heat input or account for a volumetric increase due to vaporization of a fluid.
A thermal analog of an electronic capacitor, i.e., a thermal capacitor, has a pair of similar chambers in vapor communication disclosed in a patent to Moore, Jr., U.S. Pat. No. 3,738,421. A liquid comprising a vaporizable heat transfer material and a substantially non-vaporizable solute with a relatively broad solubility range is contained in each chamber. Vaporization of the heat transfer material from one chamber and condensation in the other serves to "charge" or "discharge" the thermal capacitor. The invention to be described hereinafter avoids using a mixture of two different fluids by employing a single fluid within a single thermal energy device.